In the former East of Berlin there is an old, abandoned amusement park called Spreepark. It closed down in something like 2001 and has not been looked after since. Thieves cleaned it out pretty thoroughly. Everything but the big items is gone, and what is left is broken and/or grown over. In short, it's one of the most magical places I've ever seen. The only way to see the park (legally) is to take a guided tour or hop on a little tourist train for a 15 minute ride around the park. Sadly the park has now been sold and tours will only run through the end of April.
If you're in Berlin, make sure to catch the tour before it ends. It's like stepping into another world.